A. Field of the Invention.
This invention relates to a method of producing high molecular weight polymers and the utilization of these polymers in the production of hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores.
B. Description of the Prior Art.
Viscous aqueous solutions containing organic polymers have been utilized heretofore for carrying out a variety of treatments in subterranean hydrocarbon-containing formations to increase the production of hydrocarbons therefrom. For example, in the production of hydrocarbons from oil and gas wells which also produce large volumes of salt water, the cost of producing the salt water, separating it from the produced hydrocarbons and disposing of the salt water represents an economic loss in the production of the wells. In order to reduce the production of salt water from such wells, viscous aqueous polymer solutions have been heretofore utilized to preferentially reduce water production from portions of the producing subterranean formations. The high molecular weight polymers or copolymers in the viscous aqueous solution plug the pores of the portions of the formation producing water and thereby substantially reduce the permeability of such portions to water whereby water production is also substantially reduced.
Viscous aqueous solutions containing organic polymers have been utilized in acidizing or fracturing of a hydrocarbon-containing subterranean formation. The usual acidizing operation comprises introducing a viscous aqueous solution containing a nonoxidizing acid into the formation under sufficient pressure to allow the acid to react with acid soluble components in the formation. Fracturing involves injecting the viscous aqueous solution into the formation at a pressure sufficient to create a crack or fracture in the formation.
Viscous aqueous polymer solutions have also been utilized heretofore in processes for enhanceing the recovery of oil from subterranean oil-containing formations. In such processes, the viscous aqueous polymer solution is injected into the formation by way of at least one injection well penetrating the formation and forced through the formation towards at least one production well penetrating the formation whereby the production of oil from the formation is increased. Such processes are usually carried out in subterranean oil-containing formations after primary recovery operations are completed, but they can also be utilized during and as a part of primary recovery operations. In the usual case, during primary recovery operations, the energy required to force oil into producing wells is supplied by the natural pressure drive existing in the formation or by mechanically lifting oil from the formation through the well bores of producing wells to the surface. At the end of primary recovery operations, a substantial quantity of oil often remains in the formation.
In enhanced recovery operations, energy for producing oil remaining in a subterranean oil-containing formation is supplied by injecting liquids or gases through one or more injection wells penetrating the formation into the formation under pressure whereby the liquids or gases drive the oil to producing wells penetrating the formation. The most common of such recovery techniques is known as water flooding wherein an aqueous liquid is injected into the formation under pressure which provides the energy and flushing action necessary to force oil in the formation to one or more production wells penetrating the formation. The efficiency of such water flooding techniques varies greatly depending upon a number of factors including variability in the permeability of the formation and the viscosity of the oil remaining in the formation. When the oil remaining in the formation is of a relatively high viscosity, an aqueous flooding medium of those viscosities such as fresh water or brine tends to finger through the high viscosity oil front and thereby bypass most of the available oil. In addition, typical subterranean formations contain layers of materials which often have widely varying permeabilities to oil flow. Consequentially, the aqueous flood medium tends to follow the course of least resistance, i.e., flow through zones of high permeability in the formation and thereby bypass zones of less permeability containing oil.
In order to overcome such problems, a flooding medium having viscosities in the order of or greater than the viscosity of oils to be displaced have been utilized. Generally, the viscosity of the water-flooding medium is increased to a level at which it is close to the viscosity of the oil to be displaced, preferably equal to or greater than the viscosity of the oil to be displaced. This causes the relative flow rates of the oil and the flooding medium in the formation to be comparable and the tendency of the flooding medium to finger through the oil front or bypass oil-containing portions of the formation is substantially diminished.
Solid water-soluble organic polymers have been commonly used heretofore for increasing the viscosity of aqueous well treating and injection solutions. Generally, the water-soluble organic polymers are dissolved in the aqueous treating or injection fluid at the site, either continuously or on a periodic basis. The dissolution of solid organic polymers in aqueous solutions is difficult, time-consuming and requires special mixing equipment. In addition, the use of solid organic polymers to increase the viscosity of aqueous treatment or injection solutions has been found to involve a number of problems. In the manufacture of high molecular weight linear polymers in solid form, the polymers are formed in a solvent, usually water, and the solvent is then removed. The removal of the solvent such as by evaporation often causes undesirable crosslinking of some of the polymers. The polymer crosslinking forms insoluble particles that swell when contacted with water to form gels, commonly called microgels. Because of the presence of such microgels in aqueous polymer wells treating and injection solutions used in well treatment and enhanced oil recovery processes, the solutions tend to plug the formation into which they are injected. The removal of the solvent also promotes hydrolysis which can lead to incompatibility of the polymer with brines.
Free radical polymerization of water-soluble vinyl monomers many times produces high molecular weight water-soluble polymers only under certain conditions. For instance, the initiator systems used for the polymerization generally require an elevated starting temperature and long induction times. Typical of these conditions are starting temperatures in excess of 80.degree. F. and initiation periods of greater than 12 hours. Small changes in these reaction conditions can dramatically change the final molecular weight of the polymer. These requirements usually make the polymerization reaction unsuited for continuous polymerization processes.
Other problems are encountered in the polymerization of water-soluble vinyl monomers. For instance, if the polymerization process proceeds too rapidly, the polymers are produced at a low molecular weight which is unsuitable for oil field applications.
The present invention provides a method of producing a water-soluble polymer having a molecular weight in the range of from about 1,000 to about 10,000,000 using a free radical initiator system which is suitable for use in the recovery of hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores which overcomes or at least mitigates the above described problems.